DESCRIPTION (Investigator's Abstract): Growth cones, the growing tips of axons, are considered the pathfinding organ of the neuron. They sense many guidance cues by direct filopodial contact, but it has been unclear how the contact actually directs growth cones. This proposal focuses on a surprisingly specific aspect of contact-mediated guidance that was detected using time-lapse microscopy in dissociated cell culture. Growth cones of sensory neurons respond specifically to contact with three populations that they normally encounter in the avian embryo. Filopodial contact specifically alters the motile activity of the growth cone itself. 1) Contact with posterior sclerotome locally inhibits extension of veils. 2) Contact with Schwann cells locally stimulates protrusion of large veils and increases veil stability. 3) Contact with anterior sclerotome increases protrusion of both filopodia and veils throughout the growth cone and then locally stimulates preferential consolidation. These physiologically relevant responses are sufficient to bias the direction of travel and to mediate behaviors such as avoidance, but are more invariant than the gross behavior. They thus appear to be the component that is most relevant to pathfinding. The specificity of the responses strongly argues for specificity at the molecular level. On contact, specific ligand-receptor binding is likely to modulate distinguishable second messenger systems that cause specific alterations in cytoskeletal dynamics. To investigate each of these elements in turn, these invariant responses will be used as assays. Aim 1 characterizes cell surface molecular activities essential for two of the responses by testing candidates implicated in pilot studies. Aim 2 assesses the role of calcium and other second messenger candidates in each of the three responses. Aim 3 characterizes cytoskeletal changes on contact with each population Interactions will be optically recorded, with or without treatments with exogenous chemicals, and characterized in detail. This work will elucidate mechanisms that control axonal pathfinding, a process fundamental to human neural development.